feat: extract shop from mp/shop — initial libreshop/shop

Source moved verbatim from mp/shop/ on 2026-04-29; mp was the first
concrete adapter consuming the libreshop toolkit. Builds and publishes
git.librete.ch/libreshop/shop on every main / v* push via the standard
.gitea/workflows/build.yml shared across libreshop components.

utils/numberFormatter.ts

@@ -0,0 +1,8 @@
+export const numberFormatter = (num: number, symbol?: string): string => {
+	const formatter = new Intl.NumberFormat("de-DE", {
+		style: "decimal",
+		minimumFractionDigits: 2,
+		maximumFractionDigits: 2
+	});
+	return formatter.format(num) + (symbol ? ` ${symbol}` : "");
+};